Gas Metal Arc Welding
Gas metal arc welding (GMAW), sometimes referred to by its subtypes metal inert gas (MIG) welding or metal active gas (MAG) welding, is a semi-automatic or automatic arc welding process in which a continuous and consumable wire electrode (“welding wire”) and a shielding gas are fed through a welding gun.
FIGS. 1 and 2 illustrate the basic design of a conventional industrial GMAW system. As shown in these figures, GMAW system 10 includes electrical power source 12, wire drive assembly 14, shielding gas supply system 16, and cable assembly 18 for carrying electrical power, welding wire and shielding gas to a workpiece 20 to be welded. Wire drive assembly 14 typically includes reel stand 22 for carrying a spool 24 of a continuous, consumable wire electrode as well as drive mechanism 26 including one or more drive wheels (not shown) for driving welding wire from spool 24 through cable assembly 18 to workpiece 20. Meanwhile, shielding gas supply system 16 normally includes shielding gas source 28 and gas supply conduit 30 in fluid communication with cable assembly 18.
As illustrated especially in FIG. 2, cable assembly 18 typically takes the form of an elongated flexible cable 32 attached on one end to power source 12, wire drive assembly 14 and gas supply system 16 and on its other end to weld gun 34. As illustrated in FIG. 3, which is a radial cross-section of flexible cable 32, this flexible cable normally includes an electrical cable 34 for providing welding electrical power to the contact tip of weld gun 40, gas conduit 36 for transporting shielding gas, and flexible sheath 48 for housing the welding wire.
In practice, flexible cable 32 is normally at least 10 feet (˜3 m) long, more typically at least 15 feet (˜4.6 m), at least 20 feet (˜6.1 m), at least 25 feet (˜7.6 m), or even at least 30 feet (˜9.1 m) long, so that electrical power source 12, wire drive assembly 14 and shielding gas supply system 16 can remain essentially stationary while weld gun 34 is moved by hand to various different locations. In addition, flexible cable 32 is normally made as flexible as possible, since this provides the greatest degree of flexibility in terms of moving and positioning weld gun 34 in any desired location. So, for example, flexible cable 32 is normally made flexible enough so that it can make relatively tight bends, such as being coiled into multiple revolutions, as illustrated in FIG. 2.
In order to prevent welding wire from snagging inside flexible cable 32, the welding wire is threaded through the interior of a flexible sheath 48. Normally, this flexible sheath is made from a metal wire tightly wound in a spiral whose inside diameter is only slightly larger than the outside diameter of the welding wire, since this structure provides a high degree of flexibility in flexible cable 32 while simultaneously preventing contact between the welding wire and other components inside the flexible cable.
Because of the length and flexibility of elongated flexible cable 32, it often takes a comparatively great amount of force to drive welding wire from spool 24 through cable assembly 18 onto workpiece 20. Therefore, it is common practice in industry to coat the welding wire with a solid lubricant such as graphite, molybdenum disulfide, etc. for reducing the coefficient of friction between its external surfaces and the internal surfaces the flexible sheath through which it passes.
Submerged Arc Welding
Submerged Arc Welding (SAW) differs from GMAW in that, in SAW, no external shielding gas is used. Instead, the molten weld and the arc zone are submerged under a blanket of a granular fusible flux which, when molten, provides a current path between the electrode and the workpiece being welded, and provides protection for the weldment from the surrounding atmosphere. See, FIG. 4, which schematically illustrates SAW, FIG. 4 showing a welding wire 50 driven by drive rollers 52 forming a weld 58 in a workpiece 56, the tip of the welding wire forming an arc 54 which is totally submerged in a layer of flux 60.
A particular advantage of SAW is that high deposition rates are possible. For example, depositions rates of over 100 pounds of applied weld metal per hour (45 kg/h) or more are possible with SAW, as compared with 5-10 pounds per hour (˜2-4 kg/h) for GMAW. A particular disadvantage of SAW is that only horizontal surfaces can be welded, as a practical matter, since gravity will normally cause the granular flux to slide off of non-horizontal surfaces. Because of these constraints, SAW is normally used in applications where high deposition rate welds are made in large, horizontally-positioned objects, e.g., pipe manufacture.
FIG. 5 illustrates such a SAW system. In this system, welding wire 62 obtained from supply spool 64 is fed by means of supply assembly 66 including drive rollers (not shown) through weld gun 68 where it forms a weld on a workpiece (not shown) mounted in a stationary position on table 70. In the particular embodiment shown, supply spool 64, supply assembly 66 and weld gun 68 are mounted in fixed positions on frame 71, which is moveable along rail 72, so that a continuous weld can be formed along the entire length of the workpiece. In other embodiments, supply spool 64, supply assembly 66 and weld gun 68 are mounted in stationary positions, while the workpiece is moveable along table 70 to provide a continuos elongated weld. In either case, a suitable flux supply system (not shown) is normally provided for automatically covering the site to be welded with flux.
Depending on the application, this SAW system can be stationary, in the sense that the entire system is permanently mounted in a single use location. In other applications, this SAW system can be mobile, in the sense that the entire system can be moved between different use locations as, for example, in the welding of bridge girders.
Industrially, welding wire 62 is supplied to the weld gun 68 either on spools, as illustrated in FIG. 5, or in barrels, stems, or reels. In all cases, this wire supply is normally mounted in a fixed position with respect to, and usually fairly close to, weld gun 68. Similarly, supply assembly 66 is also usually mounted in a fixed position with respect to, and usually fairly close to, weld gun 68. As a result of these features, no guidance system such as flexible sheath 48 in the GMAW system illustrated in FIGS. 1-3 is necessary. This is because there is essentially no sliding friction between welding wire 62 and the structural elements transporting this welding wire from supply spool 64 to weld gun 68. Therefore, welding wire for SAW systems are not provided with the solid lubricants typically used in GMAW systems for reducing sliding friction.
As indicated above, deposition rates in SAW are typically much higher than in GMAW. This is made possible by using thicker welding wire and by using more electrical power than in GMAW. For example, welding wire for SAW is typically 1/16 inch (˜1.6 mm) or more in diameter while welding wire for GMAW has a diameter of about 1/16 inch (˜1.6 mm) or less. In any event, because of these substantially more intense conditions in SAW, the useful life of the contact tip of the welding gun through which the welding wire passes can be very short due to friction and abrasive wear. For example, the useful life of a standard, commercial beryllium copper contact tip when used for SAW welding with a clean (no drawing soap residue) bare (i.e., uclad) standard steel SAW welding wire can be as short as four hours or less.
Because of this problem, commercial SAW welding wire is almost always clad with a copper coating. Unfortunately, portions of this copper cladding are often stripped off the welding wire as it passes through the knruled drive rolls (e.g., drive rolls 52 in FIG. 4) and wire straighteners typically found in a standard SAW wire feeding system. Over time, this stripped copper can accumulate in the feeding system as copper flakes and may even contaminate the weld being formed. This copper contamination, in turn, can jeopardize weld integrity due a phenomenon known as “copper cracking,” which is a type of liquid metal embrittlement.